The main purpose of a belt tensioner that automatically responds to fluctuations in the movements of an endless belt is to prolong the life of the belt itself, or of engine components such as accessories operating in conjunction with the belt. Belt tensioners are typically used in front-end accessory drives in an engine. In an effort to improve the fuel economy of the engine, a vehicle may be equipped with a belt-driven starter generator (BSG). The BSG may eliminate the need for a starter motor, and may also provide low-cost hybrid capabilities such as, for example, start-stop and power assist in a vehicle.
In addition to normal alternator functions, the BSG may also act as a motor to crank the engine of the vehicle. When acting as a crank, the BSG may act as a driver pulley while a crankshaft of the engine acts as a driven pulley. Thus, because the crankshaft is actually operating as a driven pulley, the tensioning requirements of the endless belt may actually be reversed when compared to the tensioning requirements of the endless belt operating under normal conditions. However, when the BSG cranks the engine, which is with a force of great magnitude, the endless belt tends to jerk or suddenly rotate the tensioner in a direction opposite to the tensioner's normal working direction. Conventional spring-loaded tensioners currently available may not be able to resist rotation in the opposite direction when the BSG operates as the driver pulley.
In one approach to meet the tensioning requirements of the endless belt in a BSG system, two tensioners may be provided. Alternatively, a single tensioner with two arms and two pulleys may be used instead. However, the tensioning effects of two tensioners tend to cancel one another. Moreover, adding an extra tensioner or extra components tends to add cost, weight, and complexity to a vehicle. Accordingly, there is a continuing need in the art for a simple, cost-effective tensioner that provides the required tensioning when the crankshaft is operating as either a driven or a driving pulley.